1. Field of the Invention
The present invention relates to a process for the production of pentafluoroethane, a process for producing hexafluoroethane using pentafluoroethane obtained by the production process, and use of the obtained pentafluoroethane.
2. Description of the Related Art
Pentafluoroethane (CF3CHF2) is used as a low-temperature refrigerant or as an etching gas and is also used as a starting material for the production of hexafluoroethane (CF3CF3).
As for the production of pentafluoroethane, various methods have been heretofore known. For example, these are:
(1) a method of fluorinating tetrachloroethylene (CCl2═CCl2) or a fluorinated product thereof with hydrogen fluoride in the presence of a fluorination catalyst (Japanese Unexamined Patent Publication No. 8-268932),
(2) a method of subjecting chloropentafluoroethane (CF3CClF2) to hydrogenolysis (Japanese Patent No. 2,540,409), and
(3) a method of reacting a fluorine gas with a halogen-containing ethylene (Japanese Unexamined Patent Publication No. 1-38034).
When these production methods are used, various impurities such as chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFC) and hydrofluorocarbons (HFC) are contained in pentafluoroethane which is the objective substance.
In order to obtain high-purity pentafluoroethane, these impurities must be removed as much as possible. Among these impurities, various purification methods have been proposed to remove chlorofluorocarbons not only for the purpose of achieving high purity but also for preventing the depletion of the ozone layer. In particular, chloropentafluoroethane is close to pentafluoroethane in its boiling point and difficult to separate by normal distillation and, therefore, various purification methods have been proposed. For example, these are:
(1) a method by extractive distillation (Japanese International Application Domestic Publication No. 9-508626)
(2) a method of subjecting chloropentafluoroethane contained in pentafluoroethane to hydrogenolysis (Japanese Unexamined Patent Publication No. 8-301801), and
(3) a method of removing chloropentafluoroethane contained in pentafluoroethane after fluorinating it with hydrogen fluoride (HF) (Japanese Unexamined Patent Publication No. 6-256234).
Among hydrochlorofluorocarbons and hydrofluorocarbons, difluoromethane (CH2F2) and 1,1,1-trifluoroethane (CF3CH3) are known to form an azeotropic mixture or azeotropic like mixture with pentafluoroethane. These compounds are very difficult to separate from pentafluoroethane. When pentafluoroethane is produced by a method containing hydrogenolysis, 1,1,1-trifluoroethane is very often produced as a by-product due to an excess hydrogenation-dehalogenation reaction. This compound is contained in pentafluoroethane in a relatively large amount.
For removing 1,1,1-trifluoroethane contained in pentafluoroethane, a method by extractive distillation is known (Japanese Unexamined Patent Publication No. 9-12487). However, this method by extractive distillation has a problem that a plurality of expensive facilities, such as distillation towers, are necessary and the equipment cost is very high.